A backlight unit, a representative implementation example of a plane light source unit, functions to emit light uniformly to the backside of a liquid crystal display (LCD), which is not self-luminous, so as to make a displayed image visible. A light guide plate is a component that controls the backlight unit to provide uniform brightness and light emission, and is one of plastic lenses that transfers light emitted from a light source (LED) uniformly to the entire surface of the LCD.
FIG. 1 shows a structure of a conventional edge type backlight unit (BLU).
Light emitted from a light source 20 is propagated while being totally reflected through a light guide plate 30. A critical angle for total reflection is determined by refractive indexes of a medium and an air layer, based on Snell's law, and light within the critical angle is propagated while minimizing energy loss unless the light collides with a mechanical structure that makes light propagated beyond the critical angle. In the case of a printed light guide plate, scattering and transmission are caused using white dot patterns 31.
In the case of white dot patterns, light may not be used optimally due to loss of light and limit of fill factor. Therefore, in order to use the printed light guide plate as a light source, a diffuser plate 40 or a diffuser sheet should be stacked on the light guide plate 30, so that the printed white dot patterns are not seen. In addition, if a light guide plate is manufactured by an existing laser machining, a pattern processing time is long and there is a limitation in implementing fine patterns.